The invention relates to a process for the adiabatic hydrogenation of nitroaromatic compounds nitroarenes to form aromatic amines in the gas phase on fixed catalysts. In this process, the nitroaromatic reactant is passed over the catalyst under pressure and at elevated temperature with hydrogen, water, optionally nitrogen and substantially in the absence of the aromatic amine produced from the nitroaromatic reactant.
Aromatic amines are important intermediates which have to be available at low cost and in large quantities. For that reason, plants for the hydrogenation of nitrobenzene, for example, have to be constructed with very large capacities.
The hydrogenation of nitroaromatic compounds is a highly exothermic reaction. Thus, for example, at 200° C., the hydrogenation of nitroxylene to form xylidene releases approx. 488 kJ mol−1. The hydrogenation of nitrobenzene to form aniline releases approx. 544 kJ mol−1. From both an ecological and an economic perspective, the dissipation and use of the heat of reaction is an important element in the performance of processes for hydrogenating nitroaromatic compounds.
Thus in an established processing mode the catalyst is operated as a fluidized, heat-stabilized bed (U.S. Pat. No. 3,136,818). This processing mode results in effective heat dissipation but it suffers from a non-uniform residence time distribution (nitrobenzene leakage) and catalyst abrasion.
Narrow residence time distributions and low catalyst abrasion are achievable in principle in reactors having a static catalyst bed. However, problems with the temperature control of the catalyst beds frequently occur in such reactors. As a general rule, temperature-controlled multitube fixed-bed reactors are used (“isothermal operation”) which, particularly in the case of large reactors, have a very elaborate cooling circuit (DE-OS 2 201 528). Such reactors are complex and give rise to high investment costs, since the manufacture of a reactor made up of many thousands of individual tubes is very complicated and can only be performed by specialist companies at high cost. In addition, the size of the plant leads to rapidly growing problems in terms of mechanical strength and uniform temperature control of the catalyst bed which make the construction of large units of this type impracticable from a purely technical viewpoint too. Therefore, the use of this principle to construct modern plants on a world scale, which require production capacities of many hundreds of thousands of tonnes per year, is entirely unrealistic.
In contrast, simple reactors such as those suitable for conducting the process of the present invention described herein contain only catalyst beds on or between simple support grates and/or metal screens and have no system for heat balancing in the reactor. In other words, the need for complex measures to control the temperature of the catalyst beds, through the use of heat transfer oil, for example, is entirely eliminated. In this type of reactor the reaction enthalpy is reflected quantitatively in the temperature difference between the reactant and product gas stream (“adiabatic operation”). Reactors of this type are easy to transfer from a pilot-plant scale (mini-plant) to the scale of a large production plant, which considerably simplifies process development. The latter involves, for example, the fine tuning of operating parameters such as pressure, temperature, rate of flow of the reaction gases, concentration of co-reactants, etc., as well as other factors such as choice of catalyst. All of these factors must be optimized in order to achieve the greatest possible yield and selectivity. The possibility of being able to optimize these many variables in a relatively small laboratory scale test plant and then transfer the results without difficulty to a large production plant offers considerable advantages, since, for example, there is no need in principle for a pilot phase and the construction of a plant with the necessary production capacity can begin immediately. Furthermore, reactors of this type are inexpensive and robust in all sizes.
GB 1,452,466 discloses a process for hydrogenating nitrobenzene in which an adiabatic reactor is connected in series to an isothermal reactor. The majority of the nitrobenzene is reacted in a temperature-controlled multitube fixed-bed reactor. Only the hydrogenation of the residual content of nitrobenzene takes place with a relatively low hydrogen excess (less than 30:1) in an adiabatic reactor. However, the complete elimination of a temperature-controlled reactor with a purely adiabatic reaction and the associated advantages is not taught in GB 1,452,466.
DE-AS 1 809 711 teaches uniform introduction of liquid nitro compounds into a hot gas stream by atomization, preferably at constricted points directly in front of the reactor. The design of the reactor is not mentioned in DE-AS 1 809 711. It follows from the example, however, that in spite of a considerable hydrogen excess, at least 34% of the reaction enthalpy does not leave the reactor with the product gas, so the reactor is not operated adiabatically.
DE-OS 3 636 984 describes a process for the coupled production of nitro- and dinitroaromatics from the corresponding hydrocarbons by nitration and subsequent hydrogenation. Hydrogenation takes place in the gas phase at temperatures between 176 and 343.5° C. An apparatus for gas-phase hydrogenation is described which is composed substantially of two reactors connected in series with intermediate cooling and intermediate introduction of the reactant, but no mention is made of their size and design. However, it follows from the temperature profile of the reactors that a considerable proportion of the heat of reaction does not leave the reactor with the product gas stream. Thus reactor 1 has an inlet temperature of 181.7° C., a hottest point of 315.6° C. and an outlet temperature of 277.2° C.; and reactor 2 has an inlet temperature of 203.9° C., a hottest point of 300° C. and an outlet temperature of 296.7° C. No mention is made in DE-OS 36 36 984 of a cooling system for the reactors for industrial reactions of, e.g., 80,000 tonnes per year. Neither DE-OS 36 36 984 nor DE-OS 18 09 711 deals explicitly with the problem of heat dissipation in gas-phase hydrogenation reactions.
In all of the aforementioned publications, copper catalysts are used. These copper catalysts are operated exclusively with low loads (<0.1 gnitroarene/[mlcatalyst·h]) and at a low temperature level. This results in low space-time yields.
In addition to the aforementioned copper catalysts, numerous other contacts are described as suitable for the gas-phase hydrogenation of nitroarenes. They are described in many publications and include as active hydrogenation elements Pd, Pt, Ru, Fe, Co, Ni, Mn, Re, Cr, Mo, V, Pb, Ti, Sn, Dy, Zn, Cd, Ba, Cu, Ag, Au, and their compounds, in part as oxides, sulfides or selenides and also in the form of a Raney alloy and on supports, such as Al2O3, Fe2O3/Al2O3, SiO2, silicates, carbon, TiO2, Cr2O3. These catalysts too are operated only with low loads in a temperature range below 350° C.
DE-A 2 244 401 and DE-A 2 849 002 describe palladium catalysts on aluminum oxide supports which are operated as static catalyst beds in heat-exchanger tubes under normal pressure with loads of less than 1 gnitroarenes/[mlcatalyst·h] and low hydrogen/nitrobenzene ratios.
DE-A 4 039 026 describes palladium catalysts on graphite supports which are operated under similar conditions to the palladium catalysts on aluminum oxide. In all of these process variants, the large amount of heat that is generated from the reaction has to be removed from an industrial reactor by means of a complex heat exchanger system.
Only the patents EP 0 696 573 B1, EP 0 696 574 B1, EP 0 748 789 B1 and EP 0 748 790 B1 are directed to processes performed under purely adiabatic conditions. EP 0696574 B1 describes in very general terms the process for producing aromatic amines in which a gas mixture made up of nitroaromatic compounds and hydrogen is passed through the catalyst under adiabatic conditions. In the processes disclosed in EP 0 696 573 B1, EP 0 748 789 B1 and EP 0 748 790 B1, certain advantages are obtained in each case by altering various parameters.
EP 0 696 573 B1 teaches that the advantage of particularly high selectivities is achieved if the nitroaromatic reactant is passed over the catalyst with, in addition to hydrogen, a multiple of the aromatic amine produced in the reaction and a multiple of water. In this mode of operation, each catalyst volume contains at least 2 moles of amino groups and 4 moles of water per mole of nitro group. The catalysts described are the same as in EP 0 696 574 B1. The disadvantage of this processing mode is that large amounts of compounds which in principle are dispensable for the actual reaction, namely water and amine, have to be continually recycled. In particular, the constant recycling of at least 2 equivalents of the amine that is formed, in other words the valuable product of the process, is extremely disadvantageous because the amine that is produced is repeatedly exposed to high temperatures.
Patents EP 0 748 789 B1 and EP 0 748 790 B1 describe advantages obtained only through the use of specialized catalyst systems:
Palladium catalysts on graphite or graphite-containing coke with a palladium content of >1.5 and <7 mass % are disclosed in EP 0 748 789 B1. The advantage attributed to these catalysts is due to exceptionally long cycle times in comparison to all previously described catalysts. The disadvantage of this process is the immensely high catalyst cost that is inevitably associated with the high palladium concentrations. The patent does not indicate that the high catalyst costs arising from the large amounts of palladium needed for an industrial application can be offset by the long cycle times.
Palladium-lead catalysts on graphite or graphite-containing coke with a palladium content of 0.001 to 7 mass % are disclosed in EP 0748 790 B1. The advantage attributed to these catalysts is higher selectivity in comparison to analogous catalysts without the addition of lead. All of the examples described in this patent used catalysts with 2 mass % of palladium, so the disadvantage of high catalyst costs applies in this case too.
Hydrogenation in the presence of water is not taught in either EP 0 748 789 B1 or EP 0 748 790 B1.